In the St. Lawrence Lowland in New York, four lake levels and the limit of the Champlain Sea have been identified. Each reconstructed level has been traced west-southwest from Covey Hill. Water levels are correlated with previously identified levels in the Lake Ontario basin by means of an “extended” diagram. The study area includes the northwestern part of the Champlain Lowland, facilitating correlations of water bodies in the St. Lawrence and Champlain Lowlands.
The highest lake (level I) in the St. Lawrence Lowland was continuous with Lake Iroquois in the Lake Ontario basin, and it drained across Covey Hill at 329–332 m into Lake Fort Ann in the Champlain Lowland. Level II, also confluent with Lake Iroquois, drained through Covey Hill Gap at 308–311 m into Lake Fort Ann. Level III was continuous with the post-Iroquois Sydney phase in the Lake Ontario basin and with the highest phase of Lake Fort Ann in the Champlain Lowland. Level IV in the St. Lawrence Lowland was continuous with the Belleville phase in the Lake Ontario basin and with a lower phase of Lake Fort Ann in the Champlain Lowland. Level V is the limit of the Champlain Sea in the St. Lawrence and Champlain Lowlands. Projection of the reconstructed marine water surface westward indicates that the Champlain Sea initially entered the Lake Ontario basin, where it was confluent with the estuarine post-Iroquois Trenton phase. Subsequent separation of water bodies in the Lake Ontario basin and St. Lawrence Lowland occurred as a result of isostatic uplift.
Restrictions placed on ice-marginal positions by lake levels and the lake outlets indicate that the concept of contemporaneous marine and fresh-water events separated by a narrow ice dam at Covey Hill is implausible. Marine invasion of the St. Lawrence Lowland along the retreating ice sheet margin occurred subsequent to drainage of glacial lakes through the Covey Hill channels and the Champlain Lowland.